Auto merge of #1105 - bitcartel:zc.v0.11.2.z6_issue_424, r=ebfull

New private/public key pairs for broadcasting alert messages

Implements #424

Fixes and integrates method of sending alerts as described by upstream here:
- https://gist.github.com/laanwj/0e689cfa37b52bcbbb44

To send an alert:
- Copy private keys into alertkeys.h.
- Modify alert parameters and message found in sendalert.cpp
- Build and run to send the alert e.g. ./zcashd -printtoconsole -sendalert

Tested and verified with local nodes on alpha 6 testnet.

diff --cc src/Makefile.am
Simple merge

diff --cc src/init.cpp
Simple merge